1. Field of the Invention
The present invention relates generally to the deposition of ion-insertion layers on a substrate and, more particularly, to a chemical vapor deposition of thin-film materials useful in lithium rechargeable batteries. Specifically, the present invention relates to a plasma enhanced chemical vapor deposition (PECVD) method in the production of vanadium oxide thin-film layers.
2. Description of the Prior Art
In recent years, rapid miniaturization of electronic devices has greatly out-paced advances in battery technology. To reduce the size and cost of batteries, electronics manufacturers are increasingly interested in thin-film rechargeable batteries. These batteries have a unique advantage in that they can be incorporated into the same integrated circuit with other electronic elements. Since the thin layer material exhibits a lower electrical resistance, it then becomes practical to use some materials that cannot normally be used in bulk batteries because of their relatively high resistivity. Thin-film techniques also allow the exploration of the preparation of new materials. Such thin-film batteries can be used in a wide variety of applications such as gas sensors, micro-coulomb-meters, timers, micro-field-cells, capacitors, and electrochromic displays. The combination of high energy density, high specific energy and long cycle life makes thin-film rechargeable batteries a power source of choice for portable electronic devices into the next century. A more distant future application for thin-film batteries may also include electrical vehicles.
Of particular interest in this area are thin-film lithium rechargeable batteries. High capacity and stable cathodes are critical for the development of such lithium batteries. Moreover, these thin-film electrodes need to have a larger charging capacity and better cyclic stability than those of bulk electrodes. In the case of thin-film cathodes, the morphology of the film is very important to the cathode's charge capacity and cyclic stability. Appropriate preparation methods can provide a better film morphology and impart unique physical and chemical properties to the end products. Deposition temperature is one of several important factors that affects the structure and morphology of thin-films. It has been reported previously that a much better cyclic behavior was obtained for cells using cathodes prepared at lower temperatures. Many materials, such as V.sub.6 O.sub.13, V.sub.2 O.sub.5, LiMn.sub.2 O.sub.4, LiCoO.sub.2, LiNiO.sub.2, and the like, have been investigated extensively as cathode materials for lithium-ion battery applications over the previous two decades or so. Among these materials, vanadium oxide-based cathodes have certain advantages because they have a larger charging capacity than other such cathode materials.
Known chemical vapor deposition (CVD) processes have been utilized to form thin-films, and these processes generally include the following sequence of steps. First, reactant gases of specific composition and flow rate as well as carrier inert gases are released into a reaction chamber. Then, the gas species reactants are adsorbed onto a substrate. The loosely bonded atoms then migrate across the substrate and cause film forming chemical reactions. Finally, the gaseous byproducts of the reaction are desorbed and removed from the reaction chamber. The chemical reactions that lead to formation of the solid material and substrate will either be heterogeneous, that is the reaction occurs only on the surface of the substrate, or homogeneous wherein the reaction occurs in the gas phase. Heterogeneous reactions are most desirable as the reaction occurs on a heated surface and therefore can be controlled to produce good quality films. Homogeneous reactions, on the other hand, are less desirable as they form gas phase clusters of material which results in poor adhesion and low density films, as well as create particulates in the reaction chamber.
There are two basic CVD reactor types which are used to deposit films. One type is the hot wall reactor utilizing low pressures (typically one torr or less) at high temperatures (generally 600.degree. C. or greater). The other is the cold wall reactor which utilizes atmospheric pressure and a low temperature (less than 600.degree. C.). The main advantages of a cold wall reactor are its simple construction, a fast deposition rate and a low deposition temperature. The main disadvantages include poor step coverage and gas phase homogeneous nucleation. Additionally, the cold wall reactor may be of the plasma-enhanced (PECVD) type wherein the chemical reaction is further promoted by activating the gas-phase reactants with an rf voltage field, thereby creating free electrons and a plasma in the reaction chamber.
The hot wall type of reactor generally provides the deposited films with excellent purity and uniformity while maintaining conformal step coverage. However, to produce this quality of film, the hot wall reactor normally requires a high deposition temperature while the deposition rate is low. In the past, the advantages of the low pressure hot wall reactor has out weighed its disadvantages thus allowing the hot wall reactor to become the most widely used method of depositing films to date.
The PECVD method has been utilized to prepare electro-optically active transition metal oxides such as disclosed in U.S. Pat. No. 4,687,560. Moreover, this PECVD process has also been used to prepare various materials such as tantalum oxide as illustrated in U.S. Pat. No. 5,256,455, barium titanate as disclosed in U.S. Pat. No. 5,006,363, titanium as disclosed in U.S. Pat. No. 5,173,327, nitride coating of magnetic recording heads as disclosed in U.S. Pat. No. 5,466,495, tungsten as shown in U.S. Pat. No. 4,969,415, and yttrium and zirconium as shown in U.S. Pat. No. 5,260,105. On the other hand, VO.sub.2 films, which are especially useful for thermochromic applications, have previously been prepared by chemical vapor deposition at high temperatures without the introduction of a plasma. Since this prior art technique was based on CVD without plasma assistance, relatively high temperatures, i.e. approximately 300-700.degree. C., have been required to effectively produce vanadium oxide thin films.
Consequently, there remains a need for high quality vanadium oxide films for use as cathodes in lithium rechargeable batteries wherein the vanadium oxide films are produced by a lower temperature process having high deposition rates, and wherein the films have high energy density and high discharge capacity.